diff --git a/core/src/lib.rs b/core/src/lib.rs
index 13f3dcf..fd07e7f 100644
--- a/core/src/lib.rs
+++ b/core/src/lib.rs
@@ -14,4 +14,5 @@ pub mod map;
 pub mod mapgen;
 pub mod player;
 pub mod rng;
+pub mod snapshot;
 pub mod world;
diff --git a/core/src/snapshot.rs b/core/src/snapshot.rs
new file mode 100644
index 0000000..42a4b6b
--- /dev/null
+++ b/core/src/snapshot.rs
@@ -0,0 +1,109 @@
+//! Complete, versioned encoding of a world. Saves, joining players and state
+//! checksums all use it.
+
+use std::fmt;
+
+use crate::codec::{DecodeError, Reader, Writer};
+use crate::hash::fnv1a64;
+use crate::map::Map;
+use crate::player::{Player, MAX_KNOWN_PLAYERS};
+use crate::world::World;
+
+const MAGIC: [u8; 4] = *b"GRWS";
+/// Version of the snapshot format. Raise it whenever the encoded state
+/// changes.
+pub const SNAPSHOT_VERSION: u32 = 1;
+
+/// Reason a snapshot could not be restored.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum SnapshotError {
+    /// The bytes do not start with the snapshot marker.
+    NotASnapshot,
+    /// The snapshot was written by an incompatible version of the game.
+    UnsupportedVersion(u32),
+    /// The snapshot is damaged.
+    Corrupt(DecodeError),
+}
+
+impl fmt::Display for SnapshotError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            SnapshotError::NotASnapshot => write!(f, "not a snapshot"),
+            SnapshotError::UnsupportedVersion(version) => {
+                write!(f, "unsupported snapshot version {version}")
+            }
+            SnapshotError::Corrupt(error) => write!(f, "damaged snapshot: {error}"),
+        }
+    }
+}
+
+impl std::error::Error for SnapshotError {}
+
+impl From<DecodeError> for SnapshotError {
+    fn from(error: DecodeError) -> Self {
+        SnapshotError::Corrupt(error)
+    }
+}
+
+impl World {
+    fn encode_state(&self, w: &mut Writer) {
+        w.u64(self.seed);
+        w.u64(self.tick);
+        self.map.encode(w);
+        w.u8(self.players.len() as u8);
+        for player in &self.players {
+            player.encode(w);
+        }
+    }
+
+    /// Checksum of the complete world state. Two worlds with the same value
+    /// are in the same state, apart from hash collisions.
+    pub fn state_hash(&self) -> u64 {
+        let mut w = Writer::new();
+        self.encode_state(&mut w);
+        fnv1a64(&w.into_bytes())
+    }
+
+    /// Encodes the complete world behind a header that names format and
+    /// version.
+    pub fn to_snapshot(&self) -> Vec<u8> {
+        let mut w = Writer::new();
+        w.raw(&MAGIC);
+        w.u32(SNAPSHOT_VERSION);
+        self.encode_state(&mut w);
+        w.into_bytes()
+    }
+
+    /// Restores a world from bytes written by [`World::to_snapshot`].
+    pub fn from_snapshot(bytes: &[u8]) -> Result<World, SnapshotError> {
+        let mut r = Reader::new(bytes);
+        let magic = r
+            .raw(MAGIC.len())
+            .map_err(|_| SnapshotError::NotASnapshot)?;
+        if magic != MAGIC.as_slice() {
+            return Err(SnapshotError::NotASnapshot);
+        }
+        let version = r.u32()?;
+        if version != SNAPSHOT_VERSION {
+            return Err(SnapshotError::UnsupportedVersion(version));
+        }
+        let seed = r.u64()?;
+        let tick = r.u64()?;
+        let map = Map::decode(&mut r)?;
+        let count = r.u8()? as usize;
+        if count > MAX_KNOWN_PLAYERS {
+            return Err(DecodeError::InvalidValue("player count").into());
+        }
+        let mut players = Vec::with_capacity(count);
+        for _ in 0..count {
+            players.push(Player::decode(&mut r)?);
+        }
+        r.finish()?;
+        Ok(World {
+            seed,
+            tick,
+            map,
+            players,
+        })
+    }
+}
diff --git a/core/tests/snapshot.rs b/core/tests/snapshot.rs
new file mode 100644
index 0000000..204075a
--- /dev/null
+++ b/core/tests/snapshot.rs
@@ -0,0 +1,121 @@
+use grovewright_core::codec::DecodeError;
+use grovewright_core::command::Command;
+use grovewright_core::map::Pos;
+use grovewright_core::mapgen::outcrop_origin;
+use grovewright_core::player::PlayerId;
+use grovewright_core::snapshot::{SnapshotError, SNAPSHOT_VERSION};
+use grovewright_core::world::World;
+
+const ADA: PlayerId = PlayerId(0);
+
+fn rock(world: &World) -> Pos {
+    outcrop_origin(world.map())
+}
+
+/// A world at tick 12 with one player who has hit a rock once.
+fn played_world() -> World {
+    let mut world = World::new(7);
+    world.step(&[Command::Join {
+        key: [3; 16],
+        name: "Ada".to_owned(),
+    }]);
+    let pos = rock(&world);
+    world.step(&[Command::Dig { player: ADA, pos }]);
+    for _ in 0..10 {
+        world.step(&[]);
+    }
+    world
+}
+
+#[test]
+fn a_snapshot_restores_an_identical_world() {
+    let world = played_world();
+    let restored = World::from_snapshot(&world.to_snapshot()).unwrap();
+    assert_eq!(restored, world);
+    assert_eq!(restored.state_hash(), world.state_hash());
+}
+
+#[test]
+fn the_state_hash_covers_every_part_of_the_state() {
+    let base = played_world();
+    let pos = rock(&base);
+
+    let mut waited = base.clone();
+    waited.step(&[]);
+    assert_ne!(waited.state_hash(), base.state_hash(), "tick");
+
+    let mut dug = base.clone();
+    dug.step(&[Command::Dig { player: ADA, pos }]);
+    assert_ne!(dug.state_hash(), waited.state_hash(), "map and cooldown");
+
+    let mut left = base.clone();
+    left.step(&[Command::Leave { player: ADA }]);
+    assert_ne!(left.state_hash(), waited.state_hash(), "players");
+
+    assert_ne!(
+        World::new(1).state_hash(),
+        World::new(2).state_hash(),
+        "seed"
+    );
+}
+
+#[test]
+fn equal_worlds_have_equal_hashes() {
+    assert_eq!(played_world().state_hash(), played_world().state_hash());
+}
+
+#[test]
+fn a_restored_world_continues_exactly_like_the_original() {
+    let mut original = played_world();
+    let mut restored = World::from_snapshot(&original.to_snapshot()).unwrap();
+    let pos = rock(&original);
+    for tick in 0..40u32 {
+        let commands = if tick.is_multiple_of(6) {
+            vec![Command::Dig { player: ADA, pos }]
+        } else {
+            vec![]
+        };
+        assert_eq!(original.step(&commands), restored.step(&commands));
+    }
+    assert_eq!(original.state_hash(), restored.state_hash());
+    assert_eq!(original, restored);
+}
+
+#[test]
+fn bytes_that_are_not_a_snapshot_are_rejected() {
+    assert_eq!(World::from_snapshot(b""), Err(SnapshotError::NotASnapshot));
+    assert_eq!(
+        World::from_snapshot(b"GR"),
+        Err(SnapshotError::NotASnapshot)
+    );
+    assert_eq!(
+        World::from_snapshot(b"nope, not a save"),
+        Err(SnapshotError::NotASnapshot)
+    );
+}
+
+#[test]
+fn snapshots_of_another_version_are_rejected() {
+    let mut bytes = played_world().to_snapshot();
+    bytes[4..8].copy_from_slice(&(SNAPSHOT_VERSION + 1).to_le_bytes());
+    assert_eq!(
+        World::from_snapshot(&bytes),
+        Err(SnapshotError::UnsupportedVersion(SNAPSHOT_VERSION + 1))
+    );
+}
+
+#[test]
+fn cut_off_and_padded_snapshots_are_rejected() {
+    let bytes = played_world().to_snapshot();
+    assert_eq!(
+        World::from_snapshot(&bytes[..bytes.len() - 1]),
+        Err(SnapshotError::Corrupt(DecodeError::UnexpectedEnd))
+    );
+
+    let mut padded = bytes.clone();
+    padded.push(0);
+    assert_eq!(
+        World::from_snapshot(&padded),
+        Err(SnapshotError::Corrupt(DecodeError::TrailingBytes))
+    );
+}
diff --git a/docs/KONZEPT.md b/docs/KONZEPT.md
index 344464b..a2f01d2 100644
--- a/docs/KONZEPT.md
+++ b/docs/KONZEPT.md
@@ -177,7 +177,7 @@ Die eigene Spielfigur bewegt sich sofort und läuft außerhalb des Gleichschritt
 - **Prüfsummen im Spiel.** Die Rechner vergleichen laufend ihren Zustand und gleichen notfalls neu ab.
 - **Aufzeichnung.** Das Spiel zeichnet die Befehle einer Sitzung auf. Ein Fehler aus deinem Playtest lässt sich damit exakt nachspielen.
 - **Generator-Regeln.** Regeln wie „Das erste Vorkommen liegt nah am Start" werden über viele Startzahlen geprüft.
-- **Speicherstand.** Er hält Startzahl und Änderungen fest. Ein Gast bekommt ihn beim Beitritt geschickt.
+- **Speicherstand.** Er enthält den vollständigen Zustand der Welt samt Startzahl und bleibt deshalb gültig, wenn sich der Kartengenerator später ändert. Ein Gast bekommt ihn beim Beitritt geschickt.
 
 ### 7.4 Grafik und Ton